Well, it's because the lower-case "output" in your string is being matched (rather than the upper-case which is what I suspect you are expecting). You can confirm that like so: 
if ($_ =~ /^S\s+[-]*\d+[\.\d+]*\s+[-]*\d+[\.\d+]*\s*\(\s*IOPUT|OUTPUT\
+s*\)/i)
  {
    print "PRE=$`\nMATCH=$&\nPOST=$'\nIt matches with case insensitive
+...\n";
  }

[download]
Which outputs: 
PRE=  onn (bbcreccsnnl_
MATCH=output)
POST= !OUTPUT
It matches with case insensitive...

[download]
That's the easy bit :)

Getting your expression to actually do what you want it to may be a little trickier. Perhaps if you could list the "rules" for the match, I (or others) may be able to help you craft an appropriate expression.

Cheers,
Darren :)

In reply to Re: Incorrect Pattern Matching Behavior by McDarren
in thread Incorrect Pattern Matching Behavior by T.G. Cornholio

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